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SubjectRe: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup
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[Aubrey wrote]
> The idle core information is already in idle cpu mask, do we have a quick
> and cheap way to extract out?

seems hard.

since we need an intermediate cpumask memory to save the result of cpumask
bitmap operations, the only way i can get the idle core and make sure "cpus"
isn't broken is:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 361927803e31..abd844bcfb86 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6187,8 +6187,14 @@ void __update_idle_core(struct rq *rq)
*/
static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
{
- bool idle = true;
- int cpu;
+ bool idle = true, possible_idle;
+ int cpu, wb, wa;
+
+ wb = cpumask_weight(cpus);
+ cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
+ wa = cpumask_weight(cpus);
+
+ possible_idle = (wa - wb == cpumask_weight(cpu_smt_mask(core)));

if (!static_branch_likely(&sched_smt_present))
return __select_idle_cpu(core, p);
@@ -6212,7 +6218,6 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
if (idle)
return core;

- cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
return -1;
}

but the questions are:
1. is it really cheap?
2. how to use possible_idle? right now, select_idle_core() is also selecting idle cpu. we shouldn't simply return when possible_idle is false.
Thanks
Barry

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